Snow drought helped set the stage for deadly California avalanche, leading to unstable conditions

A weeklong “snow drought” in northern California’s Sierra Nevada helped set the stage for Tuesday’s deadly avalanche, after several feet of fresh snow fell on an earlier layer that had hardened, making it unstable and easy to trigger, experts said.

The new snow didn’t have time to bond with the previous layer before the avalanche near Lake Tahoe killed at least eight backcountry skiers, said Craig Clements, a meteorology professor at San Jose State University who has conducted avalanche research. Six skiers survived and rescuers were still searching for another who was still missing Wednesday.

The group was on a three-day hike in the Sierra Nevada backcountry Tuesday morning when they were trapped by the avalanche as a winter storm hit the West Coast.

Dangers are typically highest in the 24 to 48 hours after a very heavy snowfall, Clements said, and authorities have issued avalanche warnings.

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When the weather is dry and clear, as it has been in the Sierra Nevada since January, snow crystals change and can become angular or round over time, Clements said.

If fresh, heavy snow falls on the crystals, the layers often cannot bond and the fresh snow forms a so-called storm slab on a weaker layer.

“Because it’s on a mountain, it’s going to slide,” when triggered by any change in tension above or below, sometimes naturally but also because of people passing through the area, Clements said.

Authorities have not said what triggered Tuesday’s avalanche.

If there had been more consistent snowfall throughout the winter, the different layers might have bonded together more easily, Clements said. But even when a patch of snow forms, the danger often lasts only a few days while the new snow stabilizes, he explains.

Although climate change may lead to extreme weather events, including droughts and heavier precipitation, it is difficult to say how and if it will affect avalanches or where they will occur, scientists say.

Clements said this week’s avalanche was fairly typical of California’s Sierra Nevada and he didn’t think it could be linked to climate change.

Avalanches are a mechanism that determines the amount of snow that falls on weak or stable layers, and this is “a meteorological phenomenon, not a climatic phenomenon,” he said.

About 3 to 6 feet of snow has fallen since Sunday, when the group began their trip. The region was also hit by freezing temperatures and strong winds. The Sierra Avalanche Center said the threat of more avalanches persisted Wednesday and made the snowpack unstable and unpredictable.

Teams found the bodies of eight backcountry skiers near California’s Lake Tahoe and were searching for another after Tuesday’s avalanche, which authorities said was the nation’s deadliest in nearly half a century.

Six of the tour participants were rescued six hours after the avalanche.

Nevada County Sheriff Shannan Moon said Wednesday that investigators would review the decision to continue the trip despite storm forecasts.

Skiers headed to remote huts at 7,600 feet (3,415 meters) in the Tahoe National Forest on Sunday, carrying their own food and supplies. At 6:49 a.m. that morning, the Sierra Avalanche Center issued an avalanche watch for the area, indicating that large landslides were likely in the next 24 to 48 hours.

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